1. Field of the Invention
The present invention relates to an optical memory device such as an optical disk, an optical tape and an optical card for use in optical recording/reproducing apparatuses and a manufacturing method thereof.
2. Description of the Related Art
Hitherto, as a rewritable optical recording device, optical disks using a magneto-optical recording medium and a phase-change recording medium have been put into practical use. Recorded information may be erased from such an optical disk by locally raising the temperature of the optical recording medium by condensing and irradiating a light beam emitted from a semiconductor laser onto the optical recording medium. The recorded information may be then reproduced by condensing and irradiating a light beam having an intensity which will not cause erasure of the recorded information, to the optical recording medium and by discriminating the state of its reflected light. That is, the magnetizing direction is discriminated by detecting changes in the polarizing state of the reflected light in the magneto-optical recording medium and a recording mark is detected as changes in quantity of light of the reflected light in the phase-change recording medium.
In such optical disk, tracks composed of concave and convex portions called lands and groves are formed in spiral to condense and irradiate the light beam to a specific position. The track pitch is inclined to be narrowed more and more lately in order to increase the recording density of the optical disk.
The further improvement of the recording density has been discussed by realizing magnetically induced super-resolution reproduction by using a magneto-optical recording medium by electrostatically coupling a reproducing magnetic layer which is put into a perpendicular magnetizing state as temperature rises even though it is in an in-plane magnetizing state in room temperature, with a recording magnetic layer comprising a perpendicular magnetic film in Jpn. J. Appl. Phys. Vol. 35 (1996) pp. 5701-5704.
Still more, Japanese Unexamined Patent Publication JP-A 6-290496 (1994) has proposed a magneto-optical recording medium formed by laminating, one after another, a first magnetic layer whose magnetic domain wall coercivity is relatively small, a second magnetic layer whose Curie temperature is relatively low and a third magnetic layer whose domain wall coercivity is relatively large and whose Curie temperature is high, and a high density recording/reproducing method for moving domain walls existing at the boundary section of a recording mark by temperature gradient by using the magneto-optical recording medium and by detecting inversion of magnetization caused by the move of the domain walls as changes of polarizing state of reflected light. It is desirable to form the domain walls at the front boundary section and at the rear boundary section of the recording mark separately and independently in this method to stabilize the move of the domain walls and to improve the reproducing characteristic.
However, because nearly uniform magnetic film is formed in fabricating this medium by forming the: magnetic film on a substrate, domain walls are formed at the boundary edge part of a land and a groove and it has been difficult to form a magnetic film in which the front and rear domain walls are completely separated. Therefore, the front and rear domain walls have been separated by implementing a process for decomposing or eliminating the magnetic film at the track side part by annealing the both sides of the track by a high-output laser after forming the magnetic film to form a recording mark so as to straddle this processed part.
However, in the case where the track pitch is narrowed in the prior art magneto-optical disk and the prior art super-resolution magneto-optical disk, there has been a problem that thermal diffusion in the track direction becomes remarkable, thus remarkably increasing cross-write or cross-talk.
A magneto-optical disk described in JP-A 6-290496 has a problem that it requires the laser annealing process for each disk after forming the disk, thus complicating the disk manufacturing process and increasing the cost.
Hence the invention provides the following optical memory devices in order to solve the above-mentioned problems.
A first aspect of the invention provides an optical memory device comprising:
a substrate provided with a land and a groove which form an information track; and
a recording magnetic layer capable of at least optically recording and reproducing information laminated on the substrate,
part of the recording magnetic layer which forms side wall portions between the land and the groove being selectively oxidized.
A second aspect of the invention provides an optical memory device comprising:
a substrate provided with a land and a groove which form an information track;
a recording magnetic layer capable of at least optically recording and reproducing information laminated on the substrate,
part of the recording magnetic layer which forms side wall portions between the land and the groove being selectively oxidized; and
a reproducing magnetic layer capable of optically reproducing information recorded in the recording magnetic layer by transferring the information thereto.
A third aspect of the invention provides an optical memory device, comprising:
a substrate provided with a land and a groove which form an information track;
a first magnetic layer having relatively small domain wall coercivity,
part of the first magnetic layer which forms side wall portions between the land and the groove being selectively oxidized;
a second magnetic layer having relatively low Curie temperature;
a third magnetic layer having relatively large domain wall coercivity and high Curie temperature,
the first magnetic layer, the second magnetic layer and the third magnetic layer being laminated on the substrate in this order.
A fourth aspect of the invention provides an optical memory device capable of magneto-optically recording information or of reproducing the magneto-optically recorded information, comprising:
a substrate provided with a land and a groove both of which serve as information tracks;
at least one magnetic layer,
part of the one magnetic layer which forms side wall portions between the land and the groove being selectively oxidized.
In the optical memory devices mentioned above it is preferable that an angle xcex8 formed by the side wall with each of the land and the groove is within a range of xcex8xe2x89xa760xc2x0.
A fifth aspect of the invention provides a method for manufacturing the optical memory device of the fourth aspect of the invention, comprising the steps of:
forming a magnetic layer on a substrate provided with a land and a groove so as to be thick in the land and groove portions of the substrate and be thin in portions of side walls between the land and the groove;
oxidizing the magnetic layer by holding the magnetic layer in an oxygen atmosphere or oxygen plasma to form a selective oxide layer which is thin in the land and groove portions and thick in the side wall portions.
A sixth aspect of the invention provides a method for manufacturing the optical memory device of the fourth aspect of the invention, comprising the steps of:
forming a magnetic layer on a substrate provided with a land and a groove;
oxidizing the magnetic layer by holding the magnetic layer in an oxygen atmosphere or oxygen plasma;
removing the oxidized magnetic layer on the land and groove portions of the substrate by means of sputter-etching.
Because the magnetic layer on the side wall between the land and the groove is oxidized in the invention as described above, it becomes possible to suppress at least either one of cross-talk, cross-write and cross-erasure and to realize a highly dense optical memory device.
In particular, the domain wall moving type optical memory device allows the highly dense recording and reproduction by separating domain walls before and after a recording mark in the magnetic layer to move the domain walls by the oxidized side wall. It also allows the mass-productivity of the domain wall moving magneto-optical disk to be improved, thus lowing the cost.
It also allows the optical memory devices such as the domain wall moving type optical memory device to be formed without implementing the laser annealing process for each disk as described in JP-A 6-290496, thus realizing the low cost.
It becomes unnecessary to take out the disk from a sputtering apparatus to selectively oxidize during its fabrication by selectively oxidizing by using oxygen plasma after providing the selective oxide layer. It then allows the time necessary for fabricating the magneto-optical disk to be shortened and the low cost to be realized.
It is also possible to simplify the manufacturing process by removing the oxide layer other than that on the side wall part by means of etching after oxidizing the magnetic layer.